1. Field of the Invention
The present invention relates to a polyamide resin composition with an excellent whitening resistance, comprising a polyamide, as a main component, obtained by melt-polymerizing and subsequently solid phase-polymerizing a mixture of a diamine component containing 70 mol % or more of metaxylylenediamine and a dicarboxylic acid component containing 70 mol % or more of adipic acid. Also, the present invention relates to a shaped article such as a film, a sheet and a hollow container obtained by extruding or molding the resin composition.
2. Background Information
Polyamide resins (hereinafter referred to as "polyamide MXD6") obtained by a condensation polymerization of metaxylylenediamine and adipic acid have been widely utilized in manufacturing a gas barrier, multi-layered article, because they are excellent in both a gas barrier property and thermal stability at melting, and co-extrudable or co-injection moldable with another thermoplastic resin such as polyethylene terephthalate, nylon 6 (polyamide 6), polyethylene and polypropylene.
However, the polyamide MXD6 being in an amorphous non-stretched state or an amorphous low-stretched state is crystallized with whitening when heated to a glass transition temperature or higher, stored under a high humid condition, or brought into contact with water or a boiling water, thereby degrading its transparency. Therefore, the use thereof under a high humid condition or under a condition contacting with water has been limited. In particular, a polyamide MXD6 for use in the form of film or sheet, obtained by a melt polymerization and a subsequent solid phase polymerization to have a specific relative viscosity, has an even lower crystallization rate, thereby being easily crystallized with whitening. The polyamide MXD6 crystallized with whitening is reduced in its mechanical properties, especially in impact resistance.
Japanese Patent Application Laid-Open No. 4-198329 by one of the inventors discloses a polyamide composition comprising a polyamide MXD6 and another specific polyamide, such as polyamide 6, having a high crystallization rate. Although a film and sheet made of the proposed polyamide composition advantageously retain an excellent transparency even under a high humid condition, the film and sheet have a reduced gas barrier property due to the addition of another polyamide, as compared with a film and sheet made of a polyamide MXD6 alone. In addition, the sole use of a polyamide MXD6 obtained by a solid phase polymerization fails to achieve the maintenance of its transparency.
A composite material, so-called polyamide 6-clay hybrid, where clay minerals of molecular size are dispersed in polyamide 6 to ion-bond to the polyamide 6, has an increased visible-light transmittance as compared with the polyamide 6 because a growth of spherulite in the composite material is prevented by a clay layer to control the size of spherulite to the wave length of the visible light or smaller, as described in "Shin Sozai" December 1996, page 17. A similar effect is expected for the polyamide MXD6 because it is also crystallizable. To exhibit such an effect, 1% or more of clay minerals are required to be added. However, a film and sheet made of a polyamide MXD6 containing 1% or more of clay minerals are insufficient in mechanical properties such as impact strength.